In cable television it is desirable sometimes to scramble or block certain "premium" channels or pay-per-view channels so that only cable television subscribers who have made payment specifically for those services can have access thereto. It sometimes is also desirable to control subscriber access to all channels carried by a cable system.
One cable television scrambling or blocking method was disclosed in Tanner U.S. Pat. No. 4,074,311. Tanner disclosed injecting between the visual and aural carriers of a television signal a high amplitude signal which prevents a television tuner from locking onto the necessary sync pulses to properly arrange on a television screen the video information transmitted with the television signal. The scrambling method in accordance with Tanner, however, requires that each channel be jammed with an injected signal and that a "descrambling" notch filter be provided for each channel at the subscriber end of the system. Thus, multiple notch filters are required at the subscriber end and it is difficult, if not impossible, to provide subscribers real-time service changes such as in response to an on-demand pay-per-view request.
Another type of television channel scrambling known in the prior art is synchronization suppression, hereinafter sync suppression. In prior art systems implementing sync suppression, the sync restoration or timing information is typically carried by and keyed from the aural carrier of, for example, any of channels 2, 3, or 4. Thus, in prior art sync suppression scrambling systems, it was relatively easy for a cable "pirate" illegally to obtain the scrambled channels, since one could easily detect the required sync information.
Such prior art systems typically include the descrambling system in a "converter" which historically was a box connected between a customer's incoming cable line and the television receiver, for converting the cable transmitted signal for use by a standard television receiver "set." Such converter/descrambler systems are undesirable because only one channel may be viewed at a time because the converter/descrambler decodes the channels only after they are converted. And because they have the ability to decode only one channel at a time, a separate converter/descrambler is required for each television set in the home that requires CATV service.
In view of the deficiencies in prior art cable television scrambling systems, it is still desirable to have a cable television scrambling system that can effectively scramble individual television channels with reduced concern of pirating and for which there can be real-time subscriber access control.
It is therefore an object of the present invention to provide a television security system employing sync suppression with remotely addressable sync restoration.
It is another object of the present invention to provide a television scrambling system requiring only one descrambling unit able to service multiple television sets per subscriber location.
It is a further object of the present invention to provide a simple device that can be easily installed at the subscriber's location.
It is also an object of the present invention to provide a television scrambling system and descrambler that is "user friendly" with respect to VCR and remote control units and is compatible with loop or home-operated wired systems.
It is a further object of the present invention to be able to implement other sync suppression scrambling methods in conjunction with the system of the present invention.
It is still another object of the present invention to provide a service disconnect feature by which the cable system operator may disconnect the entire spectrum of cable channels from any individual subscriber as an addressable feature to require payment of late fees before continuing service will be restored.